Fibre optical parametric amplifier incorporating a Raman-pump for reduced four-wave mixing crosstalk

The Fibre Optical Parametric Amplifier (FOPA) employing highly nonlinear fibre (HNLF) has recently been highlighted as a potential technology to expand the transmission capacity of future optical communication systems. Gain bandwidths as high as 200nm [1] have been demonstrated using FOPAs, far exceeding that of the erbium-doped fibre amplifier (EDFA) at ~35nm. One limitation of the FOPA is the generation of undesired four-wave mixing (FWM) products arising from phase-matched nonlinear interactions between different combinations of input signal and FOPA-pump photons [2]. These mixing products can be spectrally co-located with the amplified signals and thus act as crosstalk, decreasing the effective optical signal to noise ratio (OSNR) and hence performance. In this paper, we experimentally demonstrate that by additionally incorporating a counter-propagating pump to provide Raman gain to the FOPA-pump within the same length of HNLF, the FWM crosstalk can be reduced in magnitude by up to 6dB compared with a standard FOPA after amplification of ten 100GHz-spaced signals and 20dB net gain. For larger numbers of channels, we envisage this improvement to be even more significant.